Lock last month for everyone, but let the finance manager still post adjustments. User Setup does both.
General Ledger Setup → Allow Posting From / To sets the open period for everyone. User Setup (Tell Me: "user setup") has the same two fields per user and they override the general ones: give the finance manager a wider range and leave the rest closed. After month-end, move the general "from" date forward and nobody posts into the closed month by accident. The year-end checklist uses the same mechanism.
User Setup also holds per-user defaults worth knowing: Register Time, Allow Posting To for each user, the Sales/Purchase Responsibility Centre, and whether the user can post without a Salesperson/Purchaser code.
